pillowfight: Histogram dump fixups

- Always dump histogram at end of workload
- Use cleaner printf rather than C++ 'iostream' interface
- Use proper header if in "Run" or "Populate" phase.

Change-Id: I8bf129780218c4306fea860d993671a4807f8b58
Reviewed-on: http://review.couchbase.org/42184
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>